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Today, I washed my hands at the kitchen sink while making lunch, and was unexpectedly chastised by my partner because she had just started the dishwasher. Apparently, she has recently come across something that has convinced her that running the hot water in your sink means the water that enters your dishwasher will be hotter (checks out), and thus the dishes will be cleaner. I tried searching startpage to see if this is real, but I couldn't make it through even one llm-slop-ticle about it before my eyes rolled back into my head, so here I am asking y'all: chat, is this real? Doesn't my dishwasher have a heating element? Is there any meaningful difference between heating the water with a hot water heater vs a dishwasher, other than I can't use my sink while the dishwasher is filling and I have to pour a bunch of cool water down the drain?

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[–] Swedneck@discuss.tchncs.de 1 points 3 days ago* (last edited 3 days ago)

Counter-question: Why do you ask this online rather than just testing and seeing if it makes a difference?

Also, does it even give bad results anyways? Because i have never once in my life encountered a dishwasher that didn't get things squeaky clean almost without even needing detergent. I suspect many people worry about the cleanliness of their dishes for absolutely no reason.

[–] nullspace@lemmy.world 8 points 6 days ago

Your dishwasher uses hot water so if you run your sink until hot water is coming out before running your dishwasher it means it's immediately getting hot water. It's about clearing the cold water out of your pipe.

Whether or not you can use the sink while the dishes are being washed depends on your water heater. If you can wash your hands while someone is taking a shower without blasting them with cold water then you should be fine to use your sink while the dishes are being washed.

[–] FatVegan@leminal.space 2 points 6 days ago

Some dishwashers have a hot and cold water intake, but it's pretty rare, abd only makes sense if you heat your water with some alternative energy source. Letting the cold water bleed out makes sti zero sense either way.

[–] Crozekiel@lemmy.zip 2 points 6 days ago

In the US, you need to run hot water before starting the machine, but once the machine is running there is no reason you shouldn't be able to use the sink as normal.

The federal government put restrictions on how much energy a dishwasher can use in a cycle, which has significantly restricted how much time they can spend on heating water and drying the dishes at the end of a cycle. In order for dish detergent to work properly, the water needs to be at least 135ºF, most machines aim to reach 140ºF. Most manufacturers will also state incoming water needs to be 115-120º minimum in order for them to be able to work (which is a common-high setting for a hot water heater - 120ºF is hot enough to scald you if you are not careful). If you do not get the cold water out of the pipes between the hot water heater and the kitchen, then the water starting temperature won't be high enough and the dishwasher won't work as well (specifically the detergent won't work as well).

[–] Widdershins@lemmy.world 2 points 6 days ago

Growing up we had to run the shower until it was hot before we could wash the dishwasher.

[–] fartographer@lemmy.world 1 points 6 days ago

Not every dishwasher has a heating element. My dishwasher does, and the manual still says to run the hot tap to make the dishwasher more effective.

Also, just because your dishwasher might have a heating element, that doesn't mean you always wanna use it. You know what the "sanitize" setting does if you accidentally leave some oily residue or food on your dishes? Everything gets some junk baked onto it. And then you (actually, me) have to scrub at hard, dried crud that you should have handled before loading the dishwasher the first time, but you were lazy and let your wife wash the dishes, and you didn't know at the time that she doesn't inspect things before baking them in the soapy dish-steamer. (Rant over)

As for washing your hands while the dish washer is running, I've never heard of this issue. Most residential plumbing is capable of carrying somewhere around 6 GPM (Gallons Per Minute), and your faucet is probably limited to 1.5 - 2.2 GPM, but if you wash your hands at that pressure, your shirt is probably gonna be soaked from splashing or spray. Your dishwasher only takes a minute or two, maximum, to fill, and the remaining 3.8 GPM is more than enough. Once your dishwasher fills, the pump (sometimes heated, sometimes not) cycles that water through the jets, and then eventually pumps the water out the drain hose.
If she's upset because you don't like using scalding-hot water to wash your hands, then that's also a non-issue. Your hot water lines are connected to your water heater, and the hot water lines will deliver water from your heater whether you want them to or not. Your cold water and hot water lines shouldn't interact behind the faucet. You can run all the cold water you want at your faucet and the only difference is that not only will the hot water supply line be hot, the cold water supply line will be cold. Separately.

Now, something you shouldn't do is run your disposal at the same time as your dishwasher. When you install your dishwasher, it specifies that you should install a "high loop." This just means to make your dishwasher's drain hose go as high as possible under the cabinet before reaching the sink drain. It doesn't have to be a literal loop, but I like making an actual loop because I like the way it looks. This "high loop" acts as a vacuum break to reduce the chances of your dishwasher sucking dirty water back into it. If you recall how to make a siphon, the two things you need are an unbroken hose with no air pockets to disturb the water flow, and for the source to be higher than the destination. Your dishwasher's drain is at the bottom, close to the floor, and the hose intersects the sink's tail piece around knee-height. The high loop makes it more difficult for backflow water to fill the cavity of the drain hose, so you'd pretty much have to clog your sink drain past the p-trap, and then fill the sink to nearly overflowing for you to move water from the sink drain to the dishwasher. BUT! This isn't true with a disposal. Disposals are capable of generating enough pressure that they can actually push some water up the dishwasher's drain hose. So, if your dishwasher is draining while you use the disposal, and it stops draining while you're still using the disposal, you might actually be able to get some of the ground up food from your disposal past the high loop. Even worse, your disposal can often let through food particles that are bigger than the jet holes in your dishwasher. Since your dishwasher fills and then pumps the water already inside it to spray your dishes, you can end up putting food particles through your pump, breaking it, or through the jets, clogging them. Also, you're taking that dank smell from your disposal and using it to clean your food.

[–] dhork@lemmy.world 127 points 1 week ago (5 children)

The Technology Connections guy on YouTube thinks so

[–] Gerudo@lemmy.zip 37 points 1 week ago (1 children)

I noticed an improvement preheating the water. It's also probably more energy efficient than letting the dishwasher do it, if it can at all.

[–] deranger@sh.itjust.works 23 points 1 week ago* (last edited 1 week ago) (2 children)

They both have electric heating elements so identical efficiency. The advantage is that priming hot water means it’s hotter for longer and reaches a higher peak temperature in the dishwasher, which means more effective cleaning.

[–] TootGuitar@sh.itjust.works 33 points 1 week ago

Not every home heats their water with electric resistance heating. My previous place used natural gas for hot water, and my current home has a heat pump water heater, which has an effective efficiency much greater than a normal electric water heater.

[–] Rhaedas@fedia.io 20 points 1 week ago (1 children)

The heating element in the dishwasher at best maintains the water at a warm temperature. It will not instantly heat the water up for the very short first cycle of prewashing, just like if you suddenly put cold water into a water heater it wouldn't be suddenly hot. It takes a lot of energy/time to warm water up.

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[–] Rothe@piefed.social 12 points 1 week ago (3 children)

If you live in the US. Dishwashers works very differently in Europe for example.

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[–] someguy3@lemmy.world 10 points 1 week ago (1 children)

Technically I think he says that IF your dishes don't already get clean.

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[–] BlameThePeacock@lemmy.ca 73 points 1 week ago* (last edited 1 week ago) (2 children)

It is real.

Here's the biggest rabbit hole you can go down. Alec has multiple videos just about dishwashers.

https://youtu.be/jHP942Livy0

You can curse me later when you've spent 2 hours learning about Christmas light colours and LEDs.

[–] Damage@feddit.it 38 points 1 week ago (1 children)

Just mind that his is a mostly American-centric view. Here in Europe my dishwasher is hooked up to the cold water. Energy usage is negligible.

[–] qupada@fedia.io 17 points 1 week ago (2 children)

European dishwashers seem to be designed vastly differently to American ones.

Water efficiency goals specifically; I've seen consumption of 7 US gallons (26.5 litres) per cycle called "eco" on American dishwashers, where mine (Miele) claims 12 litres on the heaviest cycle.

The manual for mine calls out connecting to hot water as a way to increase efficiency if you have hot water with a >1 COP, mentioning solar or heat pump cylinders.

A professional I asked noted though that for protein-heavy residues (like egg), hot water could bake it on before the dishwasher starts cleaning it.

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[–] CombatWombat@feddit.online 19 points 1 week ago (2 children)

thank you. I'm supremely annoyed that I'm reduced to posting to fedi to find good sources for a basic adulting question, but I'm equally grateful when someone comes through for me.

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[–] CubitOom@infosec.pub 53 points 1 week ago* (last edited 1 week ago) (1 children)

The technology connections guy says it helps assuming your dishwasher water connection is at the sink and that the heating element in your washer isn't very good or efficient.

Detergent packs are kinda wishy-washy (Dishwashers Explained)

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[–] lemmyng@piefed.ca 50 points 1 week ago (1 children)

Yes, as many others have said, there is an explanation for running the hot water before turning on the dishwasher.

However, your partner is wrong. The purpose of running hot water is to prime the hot water pipes. Once that's done you can go ahead and wash your hands with cold water, because the hot water pipe is already full of hot water.

[–] 4am@lemmy.zip 26 points 1 week ago* (last edited 1 week ago) (1 children)

EDIT: just to elaborate on the above comment:

You can run a mix of hot and cold water all you want, if the hot water is on, it still primes the hot water pipes with hot water; they don’t mix until they join in the faucet itself.

Running the hot water at the faucet will fill the hot water pipe with hot water (hasn’t had time to sit and cool down) so your dishwasher’s initial pre wash cycle has a higher temperature water to begin with as it draws from that same pipe; you’re not starting with slightly cooler water and waiting for the heating element to bring it up to temp (at least not as long, usually dishwasher water is slightly hotter than water heater temp).

Running cold water at the sink does not cool the dishwasher’s intake water down. Cold water isn’t mixed with hot until it’s already in the faucet (after the valves you control at the faucet itself). As long as you run the hot until hot water makes it up to the sink itself, you’re golden no matter what else is going on.

[–] hereiamagain@sh.itjust.works 1 points 6 days ago

Thanks, everyone else was missing this point and I was about to write up something similar.

[–] VibeSurgeon@piefed.social 48 points 1 week ago (4 children)

It's correct if in a country where the dishwasher lacks its own water heater.

Dishwashers in the U.S don't have water heaters and therefore it's beneficial to purge lukewarm water from the hot water line before starting the dishwasher, such that the water in the dishwasher is on average hotter and as such more effective at washing the dishes.

Dishwashers in Europe have water heaters and as such it's entirely irrelevant to do it there

[–] khannie@lemmy.world 22 points 1 week ago (9 children)

Dishwashers in the U.S don't have water heaters

Crazy. Mine only takes in cold water. Never even knew ones that take in hot water existed.

[–] VibeSurgeon@piefed.social 14 points 1 week ago (4 children)

You'll be surprised to hear that U.S washing machines have the same setup, and this is why they talk about "wash cold" or "wash hot" instead of having specific temperatures on their washing programs like 30 C, 40 C, 60 C etc.

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[–] ylph@lemmy.world 13 points 1 week ago* (last edited 1 week ago) (1 children)

I live in the US and every dishwasher I ever owned had a heating element, and was capable of heating its own water as needed, depending on what cycle was selected.

Same with my washing machines, my current one even has a steam cycle.

I feel like if dishwashers without heating elements were ever a common thing here, it must have been many decades ago.

[–] uin@lemmy.world 15 points 1 week ago (1 children)

Its not so much about that, as it is about the fact that they almost always do an initial "clean-off-the-dirtiest-bits" cycle for around 10 minutes which pulls in (mostly) cold water if your line is not purged like the OP describes. And they don't use the heating element for this initial cycle.

After the first 10 minutes they flush out the super dirty water with the dirtiest food bits and sauce and whatnot, fill in brand new water, and then run for the duration of the washing cycle (1hr plus) along with the heating element.

So flushing the cold water out of your kitchen water lines AND adding pre-wash detergent (such as some extra powder right in the door, outside of the main closable detergent compartment) helps a ton with this initial 10ish minute cycle.

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[–] cecilkorik@lemmy.ca 34 points 1 week ago (1 children)

This is primarily a North American concern. A 15amp 120V circuit does not heat water fast enough for the pre-wash cycle to actually have hot water in it. The wash cycle will eventually get hot water but not until at least halfway through, so for half of it you're washing with cold/warm water at best. Depending on what you're washing, that may be okay, but the dishwasher will definitely wash better if it is connected to the hot water supply and the hot water pipe is pre-filled with hot water by running the tap at the sink. European dishwashers are often connected to cold, because they have 220V+ heating elements that can deliver a lot more heat energy and they are designed for this. North American dishwashers are not.

The drain does not matter, you can pour whatever you want down the drain. The concern is on the supply-side only, and only applies if the dishwasher is in fact connected to the hot water line (which it should be, in North American installations, but sometimes it is installed wrong)

[–] Venator@lemmy.nz 1 points 6 days ago* (last edited 6 days ago)

you can pour whatever you want down the drain

Oh good to know! I was looking for a convenient way to dispose of all the battery acid from my dead 12V batteries 😅

[–] pineapplelover@lemmy.dbzer0.com 26 points 1 week ago (2 children)

I believe one of the technology connections videos talks about making sure the sink water is hot so the dishwasher can have hot water and not clean with cold water

It might be this video
https://www.youtube.com/watch?v=jHP942Livy0

[–] flandish@lemmy.world 3 points 6 days ago

came to post this. note: this works if your dw is on same line feed as sink. if for some reason it’s not, you can’t purge the cool water away.

One thing I just want to point out that's stated in the video is the hot water is for the pre treat cycle. It always heats up the water for the main wash cycle.

[–] CADmonkey@lemmy.world 19 points 1 week ago (1 children)

https://m.youtube.com/watch?v=jHP942Livy0

Here is a video from someone explaining why you should run the water hot before turning on the dishwasher, along with a few other tips.

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[–] neatchee@piefed.social 16 points 1 week ago

Yes. Foolishly most of the less expensive dishwashers don't heat the water themselves, and the first cycle is more effective if it doesn't have a period of using cool water from the pipes until the hot water arrives.

I was skeptical at first too but it truly works.

But it may be worth clarifying to your partner that once the water is coming out of the tap hot, the water in the hot pipes is already hot, so turning the tap to cool at that point no longer negatively impacts the dishwasher.

The important part is getting the cool water in the hot water pipes that has been sitting there flushed out before starting the dishwasher. If that's already achieved, then what you do with the tap doesn't matter anymore

[–] then_three_more@lemmy.world 16 points 1 week ago (7 children)

Is this an American thing? Dishwashers run off the cold water and heat it up themselves over here.

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[–] dual_sport_dork@lemmy.world 16 points 1 week ago* (last edited 1 week ago) (2 children)

Yes. For a typical American dishwasher installation, anyway, where the dishwasher is connected to the hot water line which is in turn connected to your storage tank water heater.

First, your water heater is already full of hot water that you've already paid the energy cost to heat up. It's therefore nonsensical to deliberately pump cold water from the line into your dishwasher only for the dishwasher to have to heat it up (using energy/money) and then have your water heater also heat up the displaced water inside it that was just pumped out.

On dumb dishwashers that don't do anything to sense the internal water temperature, this will also moderately improve your wash performance. (GE calls this feature "Automatic HotStart," for instance. Other brands call it something different. These machines will pause and run their little internal water heater coil at full blast until their current charge of water is at a temperature their programming considers acceptable to start the cycle. There is no performance detriment in this case, but it will cost you time and more energy/money.)

Modern dishwashers use a surprisingly small amount of water per cycle. Usually somewhere around 3.5 gallons for the entire thing from start to finish. This is contrary to what your parents think, who will insist to you they can totally hand wash an entire sink of dishes with "less water" than that wasteful dishwasher, which they absolutely cannot. Your dishwasher is not consuming water the entire time it is running. It will fill and recirculate that fill's worth of water inside itself, usually going through 3 or 4 phases of filling and draining throughout the cycle to arrive that that final water usage figure of around three and a half gallons. Therefore it does not readily have access to hot water unless you've primed it by running your sink, as what used to be hot water in your pipes rapidly cools off when said water is not flowing. Your dishwasher probably will not draw in enough water in one fill cycle to purge all of the standing cold water that's in the pipe unless the distance from your water heater to the dishwasher is unrealistically short.

(The latter actually is the case for the dishwasher at my office, which is fed by an undersink "low boy" water heater that's directly next to it and only separated from the dishwasher by a grand total of about 18 inches of pipe.)

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[–] HejMedDig@feddit.dk 14 points 1 week ago (1 children)

Depends on how the dishwasher is connected, but I would assume it's connected to the cold water. At least that's the common way in Scandinavia

[–] crank0271@lemmy.world 10 points 1 week ago (1 children)

That's a good point. I think in the US it's connected to the hot water line, so the answer to this probably differs by region.

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[–] DJKJuicy@sh.itjust.works 12 points 1 week ago* (last edited 1 week ago) (10 children)

Edit: okeedokee I'm wrong.

GE Appliances: Dishwasher - Correct Water Temperature

Ummm...what are all the USA people in here talking about?

I've installed and replaced my own dishwashers 4 or 5 times over the years and helped friends and family members many times. They only last about 5-7 years so it happens often enough for it to be routine for me.

The water supply is connected to the cold water. The dishwasher has it's own heating element. It heats the water to "sanitizing" levels. There's even a sanitize button.

That water gets so hot that I can't open the door mid-cycle with my reading glasses or they'll get completely fogged up.

The sanitize button is not a new development...

[–] ProjectAzar@lemmy.world 12 points 1 week ago* (last edited 1 week ago) (1 children)

They aren't all designed to connect to your cold water line. In fact, most seem to specifically instruct you to connect to your hot water intake. You are right that most in the US have water heaters, but that doesn't mean that the machine's cycles will heat the water effectively for the full duration of the wash.

Maytag, Whirlpool, GE, and LG all recommend you connect to hotwater. Bosch (EDIT: Spelled this like the drug company not the appliance company) recommends cold. Depending on your machines cycles as well, the heating element may not get the water hot enough to make effective use of the pre-wash cycle. If your machine is connected to the hot water line and you purge the cold water from the line, you will see a more effective pre-wash (and thus more effective wash) than if you let it run cold.

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